22 research outputs found
Eulerian and Lagrangian correspondence of high-frequency radar and surface drifter data : effects of radar resolution and flow components
Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Atmospheric and Oceanic Technology 31 (2014): 945â966, doi:10.1175/JTECH-D-13-00146.1.This study investigated the correspondence between the near-surface drifters from a mass drifter deployment near Marthaâs Vineyard, Massachusetts, and the surface current observations from a network of three high-resolution, high-frequency radars to understand the effects of the radar temporal and spatial resolution on the resulting Eulerian current velocities and Lagrangian trajectories and their predictability. The radar-based surface currents were found to be unbiased in direction but biased in magnitude with respect to drifter velocities. The radar systematically underestimated velocities by approximately 2 cm sâ1 due to the smoothing effects of spatial and temporal averaging. The radar accuracy, quantified by the domain-averaged rms difference between instantaneous radar and drifter velocities, was found to be about 3.8 cm sâ1. A Lagrangian comparison between the real and simulated drifters resulted in the separation distances of roughly 1 km over the course of 10 h, or an equivalent separation speed of approximately 2.8 cm sâ1. The effects of the temporal and spatial radar resolution were examined by degrading the radar fields to coarser resolutions, revealing the existence of critical scales (1.5â2 km and 3 h) beyond which the ability of the radar to reproduce drifter trajectories decreased more rapidly. Finally, the importance of the different flow components present during the experimentâmean, tidal, locally wind-driven currents, and the residual velocitiesâwas analyzed, finding that, during the study period, a combination of tidal, locally wind-driven, and mean currents were insufficient to reliably reproduce, with minimal degradation, the trajectories of real drifters. Instead, a minimum combination of the tidal and residual currents was required.I.R. was supported by the WHOI
Coastal Ocean Institute Project 27040148 and by the
WHOI Access to the Sea Program 27500036. I.R. and
A.K. acknowledge support fromthe NSF project 83264600.
A.K. acknowledges support from the Massachusetts
Clean Energy Center (MassCEC) via the New England
Marine Renewable Energy Center (MREC).2014-10-0
Ocean variability contributing to basal melt rate near the ice front of Ross Ice Shelf, Antarctica
Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 119 (2014): 4214â4233, doi:10.1002/2014JC009792.Basal melting of ice shelves is an important, but poorly understood, cause of Antarctic ice sheet mass loss and freshwater production. We use data from two moorings deployed through Ross Ice Shelf, âŒ6 and âŒ16 km south of the ice front east of Ross Island, and numerical models to show how the basal melting rate near the ice front depends on sub-ice-shelf ocean variability. The moorings measured water velocity, conductivity, and temperature for âŒ2 months starting in late November 2010. About half of the current velocity variance was due to tides, predominantly diurnal components, with the remainder due to subtidal oscillations with periods of a few days. Subtidal variability was dominated by barotropic currents that were large until mid-December and significantly reduced afterward. Subtidal currents were correlated between moorings but uncorrelated with local winds, suggesting the presence of waves or eddies that may be associated with the abrupt change in water column thickness and strong hydrographic gradients at the ice front. Estimated melt rate was âŒ1.2â±â0.5 m aâ1 at each site during the deployment period, consistent with measured trends in ice surface elevation from GPS time series. The models predicted similar annual-averaged melt rates with a strong annual cycle related to seasonal provision of warm water to the ice base. These results show that accurately modeling the high spatial and temporal ocean variability close to the ice-shelf front is critical to predicting time-dependent and mean values of meltwater production and ice-shelf thinning.The Woods Hole Oceanographic
Institution (WHOI) participation in the
ANDRILL Coulman High Program was
supported by the National Science
Foundation Office of Polar Programs
(NSF ANT-0839108) through a
subcontract from the University of
Nebraska, Lincoln (UNL 25-0550-0004-004). I. Arzeno was
supported as a 2011 WHOI Summer
Student Fellow through the NSF
Research Experiences for
Undergraduates program (OCE-
0649139). L. Padman and S. Springer
were supported by NASA grant
NNX10AG19G to Earth & Space
Research (ESR). M. Williams and C.
Stewart were supported by the New
Zealand National Institute of Water
and Atmosphere (NIWA) core funding
under the National Climate Centre,
and the Ministry of Business,
Innovation, and Employment (Contract
CO5X1001).2015-01-0
Winter atmospheric conditions over the Japan/East Sea: The structure and impact of severe cold-air outbreaks
The Japan/East Sea is a marginal sea strategically placed between the worldâs largest land mass and the worldâs largest ocean. The Eurasian land mass extending to high latitudes generates several unique winter synoptic weather features, the most notable being the vast Siberian Anticyclone that covers much of the northeast Asian land mass. The Japan/East Seaâs very distinctive winter conditions result from being on the east side of the Eurasian landmass at mid-latitudes. The resulting winter atmospheric conditions over the Sea include the mean cold air flowing off Siberia that is occasionally spiked with severe very-cold-air outbreaks. In the winter of 1999â2000, a group of Russian, Korean, Japanese, and American scientists conducted an international program to investigate the oceanography of the Japan/East Sea and its surface forcing. During this program, we made atmospheric observations with a research aircraft and ships to understand the lower atmosphere and surface air-sea fl uxes. We report here several highlights of these investigations with a focus on the dramatic severe cold-air outbreaks that occur three to five times a winter month. We start with a refresher on the physical setting and the winter mean and synoptic conditions, then describe the marine boundary layer and air-sea interaction based on research aircraft and ship measurements, and conclude with numerical model simulations that illustrate the special role of coastal topography on the surface wind fi eld and air-sea fl uxes over the Japan/East Sea
What lies beneath? Interdisciplinary outcomes of the ANDRILL Coulman High Project site surveys on the Ross Ice Shelf
Author Posting. © The Oceanography Society, 2012. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 25, no. 3 (2012): 84-89, doi:10.5670/oceanog.2012.79.Extensive field operations were conducted on the northwestern Ross Ice Shelf in Antarctica from November 2010 through January 2011. A significant amount of equipment, supplies, and people safely traversed from McMurdo Station to establish a series of combined United StatesâNew Zealand field camps at locations northeast of Ross Island. The ANDRILL (ANtarctic geological DRILLing) hot water drill system was used to melt multiple access holes through the ice shelf at each site to deploy a variety of sediment coring tools, cameras, and oceanographic instruments, as well as a remotely operated vehicle to characterize the ice shelf and sub-ice environment. These studies will contribute to future proposed geological drilling as part of the ANDRILL Coulman High Project.This work is funded by US NSF-OPP
Grant ANT-0838914 and by the NZ
Foundation for Research, Science and
Technology
CODE-2 : moored array and large-scale data report
The Coastal Ocean Dynamics Experiment
(CODE) was undertaken to identify and study
the important dynamical processes which
govern the wind-driven motion of coastal
water over the continental shelf. The
initial effort in this multi-year, multi-institutional
research program was to obtain
high-quality data sets of all the
relevant physical variables needed to construct
accurate kinematic and dynamic descriptions
of the response of shelf water
to strong wind forcing in the 2 to 10 day
band. A series of two small-scale, densely-
instrumented field experiments of approximately
four months duration (called CODE-1
and CODE-2) were designed to explore and
to determine the kinematics and momentum
and heat balances of the local wind-driven
flow over a region of the northern California
shelf which is characterized by both
relatively simple bottom topography and
large wind stress events in both winter
and summer. A more lightly instrumented,
long -term, large-scale component was
designed to help separate the local wind-driven
response in the region of the small-scale
experiments from motions generated
either offshore by the California Current
system or in some distant region along the
coast, and also to help determine the seasonal
cycles of the atmospheric forcing,
water structure, and coastal currents over
the northern California shelf.
The first small-scale experiment
(CODE-1) was conducted between April and
August, 1981 as a pilot study in "which
primary emphasis was placed on characterizing
the wind-driven "signal" and the
"noise" from which this signal must be
extracted. In particular, CODE-1 was
designed to identify the key features of
the circulation and its variability over
the northern California shelf and to
determine the important time and length
scales of the wind-driven response. The
second small-scale experiment (CODE-2) was
conducted between April and August, 1982
and was designed to sample more carefully
the mesoscale horizonta1 variability
observed in CODE-1. This report presents a
basic description of the moored array data
and some other Eulerian data collected
during CODE-2. A brief description of the
CODE-2 field program is presented first,
followed by a description of the common
data analysis procedures used to produce
the various data sets presented here. Then
basic descriptions of the following data
sets are presented: (a) the coastal and
moored meteorological measurements, (b)
the moored current measurements, (c) array
plots of the surface wind stress and near-surface
current measurements, (d) the
moored temperature and conductivity observations,
(e) the bottom pressure measurements,
and (f) the wind and adjusted
coastal sea level observations obtained as
part of the CODE-2 large-scale component.This work has
been supported by the National Science
Foundation
The 1995 Georges Bank Stratification Study and moored array measurements
The 1995 Geoges Bank Stratification Study (GBSS) was the first intensive process study conducted as part of the U.S. GLOBEC Northwest Atlantic/Georges Bank field program. The GBSS was designed to investigate the physical processes which control the seasonal development of stratification along the southern flank of Georges Bank during spring and summer. Past work suggested that during this period, larval cod and haddock tended to aggregate to the thermocline on the southern flank where higher concentrations of their copepod prey were found. A moored array was deployed as part of GBSS to observe the onset and evolution of sesonal stratification over the southern flank with sufficient vertical and horizontal resolution that key physical processes could be identified and quantified. Moored current, temperature, and conductivity (salinity) measurements were made at three sites along the southern flank, one on the crest, and one on the northeast peak of the bank. Moored surface meteorological measurements were also made at one southern flank site to determine the surface wind stress and heat and moisture fluxes. The oceanographic and meteorological data collected with the GBSS array during January-August 1995 are presented in this report. Meteorological data collected on National Data Buoy Center environmental buoys 44011 (Georges Bank), 44008 (Nantucket Shoals), and 44005 (Gulf of Maine) are included in this report for completeness and comparison with the GBSS southern flank meteorological measurements.Funding was provided by the National Science Foundation under Grant Numbers
OCE-98-06379 and OCE-98-06445
MESOSCALE CIRCULATION AND THERMOHALINE STRUCTURE OF THE BLACK-SEA OBSERVED DURING HYDROBLACK-91
Results from nearly 300 hydrographic stations occupied during HydroBlack '91, a basin-wide survey performed during September 1991, reveal a complex, eddy-dominated three-dimensional circulation in the Black Sea. Except in the areas of anticyclonic eddies distributed around the periphery of the basin, the inshore portion of the Rim Current along the relatively smooth continental margin topography forms a narrow coastal jet characterized by small amplitude (approximately 20-30 km) undulations with a wavelength of approximately 125 km. The offshore margin of the Rim Current, however, exhibits large meanders, offshore filaments and dipole eddy structures. The Rim Current is accompanied by a strong frontal zone having cross-frontal temperature and salinity differences of approximately 1.0-degrees-C and approximately 0.5, respectively, near the surface. The circulation within the interior of the sea is composed of an interconnecting series of cyclonic eddies and gyres, varying in size and shape with depth. The mesoscale variability introduces a different scale of complexity to the general circulation at each pressure level, although larger and quasi-persistent features of the general circulation are fairly coherent down to 500 dbar. Important structural changes, however, take place in both large scale and mesoscale components of the circulation at 500 dbar and deeper, consistent with the findings of the September 1990 survey (OGUZ et al., Deep-Sea Research I, 40, 1597-1612). The circulation is strongest in the upper layer (approximately 150 dbar), where the calculated geostrophic currents have speeds of 0.2-0.3 ms-1 along the axis of the Rim Current. The strength, width, and frontal intensity of the Rim Current, however, reduce considerably below the permanent pycnocline